The drill string of a conventional blast hole drill extends downwardly through an opening in the deck of a machine, and there is a tubular cylindrical bushing mounted on the deck through which the drill string passes with a relatively close fit, the bushing serving to guide the drill string and also provide a bearing and wear surface. Suck deck bushings have heretofore generally been rigidly mounted with respect to the deck, as the result of which lateral vibrations emanating from the drill string are transmitted directly to the machine.
Very substantial lateral vibrations can occur in a drill string, for example when the bit encounters hard rocks or other irregularities. As the drill string becomes longer, there can be very significant column bending of the string, and this can result in additional or magnified vibrations. Also, resonant conditions can occur at certain lengths and/or rotational speeds, again resulting in added or magnified lateral vibration. Where the bushing is rigid with respect to the deck, lateral vibrations are transmitted directly to the machine and serious structural damage can result; there are many instances where vibrations have resulted in broken welds and other structural damage to the machine. Further, undampened vibrations cause excessive wear of the drill string sections and bits, or result in wear or damage to the rotary driving head.
Considerable effort has been devoted to analyzing and overcoming the problem of lateral vibration, but no known solution is fully effective. There are, for example, guide bushings provided with rubber rings or collars bonded to their exterior surfaces. These may successfully damp vibrations, but only for a very short time since they simply do not have the requisite mechanical strength. A particular problem in this regard is that the drill string also exerts rotational and vertical forces on the bushing, and these, with the lateral movements, result in a grinding action which can destroy the rubber backing in a matter of minutes. Making the rubber harder may increase its life, but it also reduces its effectiveness in damping vibration. Further, the resilience of bushings of the noted type cannot be adjusted to meet varying conditions.